As known in the art, and as also described in my copending application Ser. No. 478,282 filed June 11, 1974, the outlying stations of such a system are each equipped with a video transmitter and a video receiver in addition to the usual telephone apparatus, these instruments being linked by audio and video lines with a common central office. Each station also includes a source of synchronizing pulses which controls the sweep circuits of its own video transmitter as well as those of its own receiver temporarily connected thereto by way of the central office. The synchronizing pulses are generated by individual crystal-stabilized oscillators operating independently of one another.
Although currently available oscillators of this type have a high frequency stability, it is nevertheless virtually impossible to keep several such oscillators in step with one another for extended periods of time. A moderately expensive crystal-stabilized oscillator may hold its rated frequency with a tolerance of 30 to 50 parts per million, corresponding to a possible deviation of 0.24 to 0.4 Hz for a line frequency of 8 kHz. The transmitting section of a station in communication with another, therefore, will have its line and frame scans controlled by locally separated sync pulses originating at the remote station, the phase relationship of the two pulse trains being generally inconstant and subject to progressive change. Since a major portion of the incoming video energy is concentrated in these sync pulses, which are separated from the accompanying picture signals by an extractor in the receiving section of the station, some of the pulse energy is unavoidably transferred to the transmitting section by an interference phenomenon which may be termed cross-vision, analogous to cross-talk, between the incoming and outgoing video lines. The randomly occurring interference pulses in the transmission section superimpose themselves on the outgoing video signals and, at the remote station, give rise to a vertical streak or bar on the screen of the television receiver, this streak traveling more or less slowly across the screen in one or the other direction according to the sense of the progressive relative phase shift between the two trains of sync pulses; with the tolerances referred to above, the streak may pass across the screen once every 2.5 to 4 seconds on the average. In an analogous manner, the outgoing sync pulses from the transmitting section of the station are partly transferred to the local receiving section where they produce a similar streak; in fact, this near-end interference phenomenon is more pronounced than the disturbance at the remote station, owing to the higher energy level of the outgoing sync pulses as compared with the incoming ones.